Photolithography involves the application of energy to photo resist deposited on a wafer. The application of energy is controlled through the use of a patterned photomask, using a step and repeat procedure where the pattern is repeated over a number of different fields. To eliminate the printability of certain defects, the pattern is reduced on the wafer using a 1/10 or a 1/5 reduction stepper. Although effective in eliminating some defects, this procedure does not eliminate all defects printed on to the wafer. Furthermore, the pattern from the photomask is repeated several times, and upon several different wafers, thus the defects of the photomask are capable of being repeated several times. Accordingly, it is important to detect and correct as many defects as possible in photomasks prior to production of the wafers.
Currently, inspecting photomasks include a number of different approaches. One approach is to inspect a photomask directly using an optical microscope. Previously, this type of inspection was done manually. However, the use of the optical microscope has evolved to automated inspection employing high resolution CCD imaging systems as the masks have become more complex. For automated photomask inspection, a die-to-database inspection is often used. The die on the photomask is compared with a design on the database used to create the photomask. In this type of inspection, a mask is illuminated from one side. An image of the mask is projected on to an image sensor, which digitizes the image. The digitized image is then directed to defect detection circuitry. The defect detection circuitry also obtains a digitized image according to the original pattern stored in the design database. The digitized pattern image also attempts to predict or model the image resulting from the exposure process. The two images are then compared for discrepancies, and the masks having discrepancies are flagged as a potential defected mask. One problem with this approach is that these programs cannot account for all of the possible variations which occur during the exposure process.
Another type of direct mask inspection technique includes a die-to-die inspection, which uses a process similar to that used in die-to-database analysis. For die-to-die analysis, two images on the mask are compared against one another. Assuming the defects are not located at the same location, the defect will be found by comparing the images of the two dies. However, both the die-to-die and the die-to-database inspection techniques have difficulty in accurately detecting defects in photomasks. As feature size continues to decrease on the photomask, conventional microscopes experience difficulty in detecting the small features, and the impact of pattern defects and optical effects increases proportionately.
Another problem with both of these techniques is that imaging of the mask conducted during inspection is very different from the imaging which is actually exposed on the wafer. During exposure of the photomask, the optical setup is quite different than that of the inspection tool. The wavelengths utilized in the exposure process and the inspection processes are different, and thus the coherence of the light is different for the two processes. The different wavelengths used in exposure and inspection can result in defects becoming difficult to detect during inspection, or creating false defects. The numerical apertures used for exposure and for inspection are also different. Therefore, defects which are detected during the inspection above process may not actually print on the wafer, and do not need to be repaired. Alternatively, some defects will not be detected and yet continue to print on the wafer.
These conventional inspection systems detect defects in the patterns themselves by inspecting the original data from which the mask is constructed, or by inspecting the mask after it is printed on the glass substrate. However, many defects are not noticeable until the feature is fleshed out in three dimensions by forming the pattern in the resist. Defects in either the mask or the resist pattern during processing can have a significant effect on the accuracy and electronic characteristics of the semiconductor device. Existing inspection methods are limited because they are unable to anticipate the defects which appear when the resist is formed on the patterned wafer. Such defects result from defects in the pattern as well as from characteristic behavior of the resist material during etching. Existing methods do not take into account the characteristics of the resist material which will be formed according to the mask pattern. As a result, a mask may be inaccurately flagged as defective when the alleged defect would not impact the final resist pattern. Alternatively, there may be a feature in the original pattern which is accurately captured in the mask pattern, but which cannot be accurately formed in the resist due to characteristics of the resist material. The mask is, as a result, inaccurately flagged defect-free when in fact, one or more defects will appear when the resist is formed according to the pattern.
Another approach to inspect a photomask is to inspect a wafer that has been shot using the photomask. A wafer is exposed using the particular photomask to be inspected, as in standard production of wafers, and then the dies on the wafer are inspected. To maintain consistency of the optical parameters during exposure and the inspection process, the wafer is exposed using the same stepper used in standard production. The wafer is then viewed under high magnification to locate defects on the wafer. After the location of these defects is determined, the defects are traced back to the photomask. For repeating defects, this approach can be quite effective. However, as feature size continues to decrease, current inspection tools are limited to certain resolutions. Therefore, some features and defects, are difficult, or sometimes impossible to view using current wafer inspection tools.
Accordingly, what is needed is a better, more reliable approach to inspect wafer defects. What is further needed is a way to inspect wafers incorporating smaller feature sizes using current inspection machines.